In MOS transistors, the progressive miniaturization is accompanied by correspondingly short gate channel lengths of the field-effect transistors. With the aid of a so-called halo implantation, in new process generations of MOSFETs, it is generally attempted to enable a relatively stable threshold voltage of the transistor despite the short gate channel lengths. However the transistor threshold voltage nevertheless falls sharply in the case of channel lengths in the range of between approximately 50 nm and 100 nm.
In technological terms, a halo implantation is currently a doping dose which is implanted under the gate after the gate patterning at a largest possible angle and increases the channel doping of the gate channel directly adjoining the source and drain regions of the FET. This effectively means that a higher channel doping is effective in the case of transistors having a short channel length than in the case of longer transistors. This counteracts the sharp fall in the transistor threshold voltage, the so-called “roll-off effect”, which occurs in the case of transistors without a halo implantation. This means that the threshold voltage in the case of short transistors with a halo implantation decreases continuously, and not abruptly.
One disadvantage of a customary halo implantation consists in the restriction of the implantation angle, which is determined by the distance between adjacent gate structures and the height of the gate structures. Moreover, the halo implantation is also effected into the entire source/drain region, as a result of which a parasitic junction capacity is undesirably increased significantly. What is more, there is currently no possibility technologically for spatially delimiting the doping, produced by the halo implantation, only closely beneath the gate oxide. In the case of known methods for fabricating short gate channels, the halo doping is implanted after the gate patterning. The disadvantages described above cannot be avoided in this way.